Trigonal polymorph of Li₂MnO₃

We report the discovery of a trigonal polymorph of the prospective Li ion battery material Li₂MnO₃ and its synthesis in bulk, single crystal form. Crystal growth of trigonal Li₂MnO₃ is strongly dependent upon the quality of a polycrystalline LiMnO₂ precursor consumed in the synthesis process. The crystal structure of the new trigonal phase is composed of ordered honeycomb layers of LiO₆ and MnO₆ octahedra segregated by layers of LiO6 octahedra and represents an ordered stacking variant of the known monoclinic polymorph. Diffuse reflectance spectroscopy reveals a direct optical gap of 2.47 ± 0.11 eV and a series of charge excitations that are well explained by the expected Mn⁴⁺ 3d³ valence. Density functional theory calculations are in excellent agreement with the spectroscopic measurements and find a near degeneracy in the formation energies of the two polymorphs. Furthermore, our results suggest that the trigonal structure resolves the compositional and structural disorder often manifested in the monoclinic phase.